Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Compositions to be polymerized by wave energy wherein said...
Reexamination Certificate
2000-11-17
2002-06-11
Berman, Susan W. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Compositions to be polymerized by wave energy wherein said...
C522S168000, C522S173000, C522S175000, C522S178000, C522S181000, C522S182000, C522S188000, C526S263000, C526S270000, C526S286000, C526S302000, C526S304000, C526S328000, C526S333000
Reexamination Certificate
active
06403674
ABSTRACT:
BACKGROUND OF THE INVENTION
Superabsorbent polymers are capable of absorbing many times their own weight in aqueous solutions. Therefore, there are many uses for polymers including use in infant diapers, adult incontinence products, feminine hygiene products, paper towels, surgical sponges, meat trays, disposable mats for outside doorways and bathrooms, household pet litter, bandages and wound dressings, controlled drug delivery, humidity-controlling products, soil conditioners, controlled release of fertilizers, thickening agents for cosmetics to concrete, sealing of underground cables, artificial snow, sensors, aqueous waste management, and gelling agents.
Due to their wide array of uses, large amounts of superabsorbent polymers are synthesized every year. It is estimated that 901,000 metric tons of superabsorbent polyacrylate polymers were produced in 1998.
A conventional method of manufacturing superabsorbent polyacrylate polymers has involved dissolving an acrylate monomer and a cross-linking monomer in a common solvent thereby forming a solution. The solution was then polymerized to form the cross-linked polymer. While this process is acceptable for superabsorbent polyacrylate polymers that can be formed from monomers with a common solvent, this process does not allow for the production of superabsorbent polyacrylate polymers made from monomers in which at least one of the monomers could not be adequately dissolved in a desired common solvent. The lack of adequate solubility of either monomer in the desired solvent has prevented the production of some superabsorbent polyacrylate polymers with desirable characteristics.
Thus, there is a need for a method of manufacturing superabsorbent polymers that will enable the use of monofunctional monomers and multifunctional monomers that lack a common solvent. Advantageously, by eliminating the solvent, the cost of the manufacturing process will be decreased and the environmental risk associated with the use of a solvent will be avoided. Furthermore, the wide range of articles that utilize superabsorbent polymers is indicative of the ever present need for a new process that will enable the production of superabsorbent polymers with unique properties that could be utilized in new applications or provide even better performing superabsorbent polymers in existing applications.
SUMMARY OF THE INVENTION
The present invention is a method for synthesizing superabsorbent polymers comprising the following steps. Providing a monofunctional monomer and a multifunctional monomer which are soluble in one another. Mixing the monofunctional monomer and multifunctional monomer in the absence of a solvent to form a solvent free solution. Exposing the solution to a source of energy that will initiate free radical polymerization thereby creating a cross-linked polymer. The product created by this method is also a part of the present invention.
These and other features, advantages and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification and claims.
DEFINITIONS
Herein, the term “solvent” shall mean a substance, other than a monomer, which is capable of dissolving one or more monomers. The term solvent is defined to include a diluting agent.
A “solvent free” solution shall mean that no solvent, as the term “solvent” is defined above, is intentionally added to the subject solution.
A “monofunctional monomer” is defined as a monomer that contains only one unsaturated carbon to carbon bond that can participate in free radical polymerization. Acrylic acid is an example of a monofunctional monomer.
A “multifunctional monomer” is defined as a monomer that contains two or more unsaturated carbon to carbon bonds that can participate in free radical polymerization. Triethylene glycol divinyl ether is an example of a multifunctional monomer.
The term “soluble” is defined to mean the condition in which a first material is dissolved in a second material such that a solution is formed. As used herein, the first material is soluble in the second material if the first material readily dissolves in the second material without excessive use of heat, pressure or physical agitation.
The term “common solvent” is defined to mean a solvent, as the term solvent is defined above, in which both the monofunctional monomer and the multifunctional monomer are soluble.
A “superabsorbent polymer” is a cross-linked polymer that is capable of readily absorbing at least fifty percent of its own weight in water. Carpol™ C940, a commercially available cross-linked polyacrylate polymer, manufactured by B.F. Goodrich Company, is an example of a superabsorbent polymer.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENT
In general, the present invention is a method of making superabsorbent polymers by mixing one or more monofunctional monomers with one or more multifunctional monomers in the absence of a solvent to form a solvent free solution which is then exposed to a source of energy that initiates a free radical polymerization process thereby resulting in the creation of a cross-linked polymer. To facilitate the polymerization process, a free radical initiator may be added to the solution of monomers prior to exposing the solution to the source of energy. The monofunctional monomers and the multifunctional monomers are selected so that they are soluble in one another. A typical source of energy that can be used to initiate the free radical polymerization is ultraviolet (UV) light. The molar ratio of monofunctional monomer to multifunctional monomer is from about 0.0001:100 to about 100:0.0001. Preferably, the combined weight of the monomers is at least 90% of the solution's total weight. More preferably, the combined weight of the monomers is at least 95% of the solution's total weight. Most preferably, the combined weight of the monomers is at least 99% of the solution's total weight. Typically, the solution is deoxygenated and then cast onto a substrate as a thin liquid film. The thickness of the film needs to be controlled so that the heat generated during the exothermic polymerization process is allowed to dissipate without harming the polymer. The use of a casting substrate that also functions as a heat sink is one way to accelerate the flow of heat away from the monomers during the polymerization process thereby allowing for the synthesis of a thicker polymer film than would be feasible if a heat sink was not used as the casting substrate. After casting, the film of solution is then exposed to UV light for a very short time, preferably less than ten seconds. The exposure to UV light may take place in a low oxygen environment. A cross-linked network forms as a solid film that will readily absorb aqueous solutions. The film that forms can be left intact or ground for use as a powder. The absorption capability of the polymer thus formed depends on the chemistry of the monomers used and the molar ratios of monofunctional monomer to multifunctional monomer. Furthermore, the process can be adjusted by varying the amount of initiator, the intensity and/or length of time the solution is exposed to the source of energy and/or the amount of oxygen in the solution.
Many different monofunctional monomers and multifunctional monomers can be used to customize properties of the superabsorbent polymer. In particular, the method of the present invention allows the use of monofunctional monomers and multifunctional monomers that are soluble in one another but are not both adequately soluble in a common solvent which is capable of sustaining free radical polymerization. Hence, the method of the present invention allows the combinations of monofunctional monomers and multifunctional monomers previously considered not feasible due to the lack of an acceptable common solvent. Examples of monofunctional monomers that can be used with the present invention include acrylate monomers, methacrylate monomers and vinyl monomers. Examples of acrylate monomers include acrylic acid, 2-hydroxyethyl acrylate, a
Berman Susan W.
Eveready Battery Company Inc.
Fraser Stewart A.
Welsh Robert W.
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